Grease composition

ABSTRACT

Grease composition for use in resin lubrication incorporating into a grease base material which includes a base oil and a fatty acid metal salt thickener at least one saturated or unsaturated fatty acid having from 8 to 22 carbon atoms and/or fatty acid metal salt, being a metal salt of a linear saturated fatty acid having from 8 to 14 carbon atoms or a metal salt of an unsaturated fatty acid having from 16 to 22 carbon atoms and from 1 to 4 unsaturated groups, the metal having a valence of from 1 to 4 excluding fatty acid metal salts used for the thickener. The grease composition of the present invention gives satisfactory lubrication properties between resin and resin or resin and other material such as a metal.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a grease composition for use in resin lubrication which is used at lubrication points where rolling or sliding occur when resin materials are employed.

BACKGROUND OF THE INVENTION

In recent years, the use of resin materials for components in various industrial machines, not least in the automotive industry, has become prominent from many points of view, such as lighter weight, lower cost, lower friction, or recycling. But as the structural elements of the components have diversified, many new problems have emerged, and all sorts of technologies are being improved.

For example, in the movable parts of electric door mirrors or the sliding parts of telescopic shafts in car steering, the various sliding parts of R&P steering rack guides, the power transmission gears of electric power steering devices, the internal sliding parts of various actuators and air cylinders, linear guides and ball screw retainers in machine tools, various bearing retainers, the sliding parts of crane booms, and also the resin gear parts in audio machines such as radio cassette players, videotape recorders and CD players, the resin gear parts in office automation equipment such as printers, photocopiers and faxes, and the sliding parts in various electrical switches, there are lubrication points where resin and resin or resin and a material other than resin such as a metal function by coming into a state of contact.

Hitherto, in the field of lubrication almost all the structural elements of machines were of metallic materials, and so the history of research into friction and wear of metal against metal such as iron, aluminium, alloys thereof, brass and bronze is old and vast, and many techniques have been accumulated through this profound experience and knowledge.

For example, it is known that extreme pressure agents and anti-wear agents which contain elements such as phosphorus or sulphur are effective against the friction and wear of metal against metal, and that these additives form a film which proactively causes a chemical reaction with the metallic surface, thereby exhibiting the functions of reducing friction and wear and preventing machinery from seizing up. This technology is widely used in engine oils and gear oils and in high-performance industrial lubricating oils and greases.

However, despite the fact that the history of the technology of lubrication of resins against resins, or resins against different materials such as metals, is brief, as mentioned above their applications have broadened in recent years, but the present situation is that in the course of this diversification no technology has been presented that is able thoroughly to satisfy the various requirements imposed on lubricating greases.

For example, in the case where technology using phosphorus or sulphur additives effective against friction or wear in the aforementioned metal-to-metal cases is applied to lubrication points of resins and metallic materials, virtually no friction reduction effect such as can be obtained for metal-to-metal is obtained. In fact there are by no means a few instances where the friction and anti-wear performance deteriorates and the life of the machine component is shortened.

This is thought to be because the chemical activity of the surfaces is much weaker in the case of resins than for metals, and there is virtually no reaction at the rubbing surfaces with organic additives such as those based on phosphorus or sulphur, and given that adsorption is also weak, the effect in regard to friction and wear is paltry, and accordingly the friction reducing effect is weak. Also, in cases where they are used at boundaries where temperature rises are deliberately effected, the active sulphur and phosphorus in these additives permeate into the resin components and cause cracks and brittleness. There are also cases where the contrary actions of friction and wear are promoted.

In order to improve the lubrication state of the aforementioned resins against resins or resins against different materials such as metals, a grease has been proposed (Japanese Patent H6-43594 (1994)) for use in plastics lubrication which contains in a lithium grease a mixture of alkylene oxide-polyhydric alcohol addition polymerisation oligomers and chained hydrocarbon oligomers, with a quaternary ammonium salt-containing clay mineral with a dispersant. Also, a technology has been disclosed (Japanese Laid-open Patent 2005-54024) for a grease composition for use in resin lubrication which contains a non-polar wax or polar wax along with a base oil such as a poly-α-olefin oil, a mineral oil or a highly refined mineral oil and a metallic soap or metallic complex soap thickener. Further improvements are anticipated.

This invention is an attempt to obtain a grease composition for use in resin lubrication wherein friction is attenuated and satisfactory lubrication performance is obtained at lubrication points where rolling and sliding and so on occur where at least one side of a paired structure, such as resin against resin or resin against a different material such as a metal, is comprised of a resinous material.

Having undertaken research and investigations on the basis of the theory of surface chemistry into the lubrication behaviour of resins in the past, the inventors have arrived at this invention by discovering that the extremely weak electrical charge that occurs at the surface of a resin and a paired material, such as resin against resin or resin against a different material such as a metal, interacts with certain saturated or unsaturated fatty acids and fatty acid metal salts which are added to greases, and further that these additives exhibit a binder function with the grease so that it is possible to form and maintain more reliably a lubrication film on the surface of the resin and resin or paired material, with the result that friction is reduced and satisfactory lubrication is obtained.

SUMMARY OF THE INVENTION

This invention is for a grease composition for use in resin lubrication incorporating into a grease base material which includes a base oil and a fatty acid metal salt thickener at least one saturated or unsaturated fatty acid having from 8 to 22 carbon atoms and/or fatty acid metal salt, being a metal salt of a linear saturated fatty acid having from 8 to 14 carbon atoms or a metal salt of an unsaturated fatty acid having from 16 to 22 carbon atoms and from 1 to 4 unsaturated groups, the metal having a valence of from 1 to 4 (excluding fatty acid metal salts used for the thickener).

The metals of the fatty acid metal salts include metals such as lithium, sodium, potassium, magnesium, calcium, zinc, aluminium and lead.

It is preferred that the total amount of saturated or unsaturated fatty acid and/or fatty acid metal salt used is in the order of from 0.1 to 10% by mass. Further, it is possible to use, together with the fatty acid metal salt thickener, other kinds of thickener such as a urea, bentonite, calcium phosphate or sodium terephthalamate, singly or in mixtures.

According to this invention friction is attenuated so that it is possible to obtain satisfactory lubrication performance at lubrication points such as rolling and sliding points between parts comprising resinous materials one against another, and it is possible to use it over a wide range as a grease composition for use in resin lubrication.

DETAILED DESCRIPTION OF THE INVENTION

The base oil in this invention is one which may ordinarily be used as the base oil of a lubricating oil or as the base oil of a grease, and there are no special restrictions. As examples mention may be made of mineral oils, synthetic oils, animal and plant oils, and mixtures thereof.

In particular it is possible to use, singly or as mixtures, base oils which belong to Group I, Group II, Group III, Group IV and so on of the API (American Petroleum Institute) base oil categories.

Group I base oils include, for example, paraffinic mineral oils obtained by a suitable combination of refining processes such as solvent refining, hydrorefining, and dewaxing in respect of lubricating oil fractions obtained by atmospheric distillation of crude oil.

Group II base oils include, for example, paraffinic mineral oils obtained by a suitable combination of refining processes such as hydrorefining and dewaxing in respect of lubricating oil fractions obtained by atmospheric distillation of crude oil. Group II base oils refined by hydrorefining methods such as the Gulf Company method have a total sulphur content of less than 10 ppm and an aromatic content of not more than 5% and so are suitable for this invention.

Group III base oils and Group II+ base oils include paraffinic mineral oils manufactured by a high degree of hydrorefining in respect of lubricating oil fractions obtained by atmospheric distillation of crude oil, base oils refined by the Isodewax process which dewaxes and substitutes the wax produced by the dewaxing process with isoparaffins, and base oils refined by the Mobil wax isomerisation process. These too are suitable for use in this invention.

Concrete examples of synthetic oils include polyolefins, polyoxyalkylene glycols such as polyethylene glycol or polypropylene glycol, esters such as di-2-ethylhexyl sebacate or di-2-ethylhexyl adipate, polyol esters such as trimethylolpropane esters or pentaerythritol esters, perfluoroalkyl ethers, silicone oils, polyphenyl ethers, and so on.

The aforementioned polyolefins include polymers of various olefins or hydrides thereof. Any olefin may be used, and as examples mention may be made of ethylene, propylene, butene and α-olefins with five or more carbons. In the manufacture of polyolefins, one of the aforementioned olefins may be used singly or two or more may be used in combination. Particularly suitable are the polyolefins called poly-α-olefins (PAO). These are base oils of Group IV.

GTL (gas to liquid) base oils synthesised by the Fischer-Tropsch method of converting natural gas to liquid fuel have a very low sulphur content and aromatic content compared with mineral oil base oils refined from crude oil and have a very high paraffin constituent ratio, and so have excellent oxidative stability, and because they also have extremely small evaporation losses, they are suitable as base oils for this invention.

As typical examples of animal and plant oils mention may be made of castor oil and rape-seed oil.

The various aforementioned oils may be used singly or in mixtures for the base oil. The aforementioned examples are listed singly but the invention is not limited thereby.

The thickener in this invention uses fatty acid metal salts. These fatty acid metal salts are those in which the fatty acid and metal are bonded, and they are normally called metallic soaps. As illustrative examples mention may be made of lithium soaps, sodium soaps, potassium soaps, magnesium soaps, calcium soaps, barium soaps, aluminium soaps, zinc soaps, lead soaps and complex soaps therefrom. These may be used singly or in mixtures thereof.

In specific cases it is also possible as appropriate to use in combination, apart from the fatty acid metal salt thickener, other thickeners such as bentonite, clay, silica, tricalcium phosphate, calcium sulphonate complexes, ureas and sodium terephthalamate.

The additive added to the grease base material which incorporates the aforementioned base oil and thickener is a saturated or unsaturated fatty acid having from 8 to 22 carbon atoms and/or a fatty acid metal salt being a metal salt of a linear saturated fatty acid having from 8 to 14 carbon atoms or a metal salt of an unsaturated fatty acid having from 16 to 22 carbon atoms and from 1 to 4 unsaturated groups, the metal having a valence of from 1 to 4 (excluding fatty acid metal salts used for the thickener).

As examples of fatty acids forming the starting material of the aforementioned saturated or unsaturated fatty acids and fatty acid metal salts in this invention, mention may be made of caprylic acid, pelargonic acid, capric acid, lauric acid, linderic acid, myristic acid, tsuzuic acid, physetoleic acid, myristoleic acid, pentadecylic acid, palmitic acid, palmitoleic acid, margaric acid, stearic acid, 12-hydroxystearic acid, petroselinic acid, oleic acid, elaidic acid, vaccenic acid, linolic acid, linolenic acid, elaeostearic acid, tuberculostearic acid, arachidic acid, eicosadienic acid, eicosatrienic acid, arachidonic acid, behenic acid, lignoceric acid, nervonic acid, hexadocosanic acid, octadocosanic acid and erucic acid.

The saturated or unsaturated fatty acids in this invention are preferably those having from 8 to 22 carbon atoms, and in the case of fatty acid metal salts are preferably linear saturated fatty acid salts having from 8 to 14 carbon atoms or unsaturated fatty acid metal salts having from 16 to 22 carbon atoms.

If the metals in the fatty acid metal salts of this invention are lithium, sodium, potassium, magnesium, calcium, zinc, aluminium, lead and so on, the effect of reducing the frictional force between materials at the lubrication points between the resin and material other than resin is large, and these metals and fatty acids can be reacted easily. The fatty acid salts are also stable chemically and are easy to maintain in the preferred lubrication state.

As to the total amount of the saturated or unsaturated fatty acids or the one or more fatty acid metal salts, it is best to add these in an amount in the range of from 0.1 to 10% relative to the total amount of the grease composition, and preferably they should be used in the range of from 1 to 5% by mass. If they are present in an amount of less than 0.1% by mass, the electrochemical action on the surface is too small, and the effect of reducing the friction coefficient is too low. If they are present in an amount of greater than 10% by mass, it becomes difficult to demonstrate the basic performance of the grease composition (for example, viscoelasticity, shear stability, heat resistance and so on) effectively, and it is likely that it will become difficult to maintain a stable state over the long term. Costs also rise.

Also, it is possible to add as appropriate to the grease composition of this invention anti-oxidants, rust preventatives, oiliness agents, extreme pressure additives, anti-wear agents, solid lubricants, metal deactivators, polymers and other additives.

The anti-oxidants include, for example, 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-paracresol, P,P′-dioctyldiphenylamine, N-phenyl-α-naphthylamine, phenothiazines and so on.

The rust preventatives include paraffin oxide, carboxylic acid metal salts, sulphonic acid metal salts, carboxylic acid esters, sulphonic acid esters, salicylic acid esters, succinic acid esters, sorbitan esters and various amine salts.

The oiliness agents, extreme pressure additives and anti-wear agents include, for example, sulphurised zinc dialkyl dithiophosphates, sulphurised zinc diaryl dithiophosphates, sulphurised zinc dialkyl dithiocarbamates, sulphurised zinc diaryl dithiocarbamates, sulphurised molybdenum dialkyl dithiophosphates, sulphurised molybdenum diaryl dithiophosphates, sulphurised molybdenum dialkyl dithiocarbamates, sulphurised molybdenum diaryl dithiocarbamates, organic molybdenum complexes, sulphurised olefins, triphenylphosphates, triphenylphosphorothionates, tricresylphosphates, other phosphate esters and sulphurised fats and oils.

The solid lubricants include, for example, molybdenum disulphide, graphite, boron nitride, melamine cyanurate, PTFE (polytetrafluoroethylene), tungsten disulphide, mica, graphite fluoride and so on.

The metal deactivators include N,N′-disalicylidene-1,2-diaminopropane, benzotriazole, benzoimidazole, benzothiazole, thiadiazole and so on.

Since, in this invention, it is possible to reduce friction and obtain satisfactory performance at lubrication points where rolling and sliding are evident between materials where one of the pair of materials is constituted of resin, one of the paired materials must be a resin, but the part which pairs with this resin can be, in addition to a resin, not only various metallic materials such as iron, copper, aluminium or other metal, or alloys, thereof, but also rubber and glass, or non-polar materials such as ceramics, and so it can be widely used with no special restrictions.

Also, it is possible to any use ordinary plastic or engineering plastic for the aforementioned resin materials, and as examples mention may be made of polyamides, polyacetals, polycarbonates, polyethylene terephthalates, polybutylene terephthalates, polybutylene naphthalates, polyphenylene ethers, polyphenylene sulphide, fluorinated resins, polyacrylates, polyamidimides, polyether imides, polyether ether ketones, polysulphones, polyether sulphones, polyimides, polystyrenes, polyethylenes, polypropylenes, phenol resins, AS resins, ABS resins, AES resins, AAS resins, ACS resins, MBS resins, polyvinyl chloride resins, epoxy resins, diallyl phthalate resins, polyester resins, methacryl resins, and ABS/polycarbonate alloys, but they are not limited to these.

EXAMPLES

The invention is explained in detail below by means of examples and comparative examples, but the invention is in no way limited by these examples.

The following materials were prepared for the examples and comparative examples.

1. Base oil A: a mineral oil with kinematic viscosity at 40° C. of 101.1 mm²/s. 2. Base oil B: a poly-α-olefin oil with kinematic viscosity at 40° C. of 31.2 mm²/s. 3. Base oil C: a highly refined oil with kinematic viscosity at 40° C. of 47.08 mm²/s, viscosity index of 146, % CA of less than 1, % CN of 11.9, and % CP of not less than 85. 4. Thickener A: lithium 12-hydroxystearate soap obtained by a reaction of 12-hydroxystearic acid and lithium hydroxide in the base oil. 5. Thickener B: a diurea obtained by a synthesis reaction of 2 mol of octylamine and 1 mol of MDI (4,4′-diphenylmethanediisocyanate) in the base oil. 6. Thickener C: bentonite obtained by gelation after swelling bentonite with an organic solvent in a base oil. 7. Thickener D: sodium terephthalamate obtained by reaction of methyl N-octadecyl terephthalamate and sodium hydroxide in the base oil. 8. Thickener E: obtained by gelation after swelling an hydroxyapatite/tricalcium phosphate composite as expressed by [Ca₃(PO₄)₂]₃.Ca(OH)₂ with an organic solvent.

The greases were prepared in a kettle using the base oils and thickener in the proportions shown for Examples 1 to 13 in Tables 1 to 3, and the grease compositions were obtained by adding the various fatty acids and/or fatty acid metal salts.

In specific detail, in the case of the greases which used thickener A (a lithium soap) in Examples 1 to 8, the base oil, thickener and the various fatty acids or fatty acid metal salts which are the additives were first weighed out in the proportions shown in Tables 1 to 2 so that the total amount of the grease composition would be 1000 g. Then, the base oil and the 12-hydroxystearic acid and lithium hydroxide with a small amount of water were pasted into a kettle of capacity 3 kg specially used for preparation of greases. After sealing, while heating and stirring, a saponification reaction was effected, and the pressure was raised to 0.35 MPa at approximately 150° C. Then the water was gradually removed, and the contents were dissolved by further heating to 215° C. Cooling was then effected at a fixed rate, and once the soap fibres had been allowed to grow, the aforementioned additives, the fatty acids or fatty acid metal salts, were pasted in and, after a homogeniser treatment, the grease compositions for use in resin lubrication for each of Examples 1 to 8 were obtained.

The fatty acid metals salts specified in Tables 1 to 3 were used as obtained by first reacting the fatty acids and metals in accordance with the molar ratios specified in Tables 1 to 3 (and similarly also for the comparative examples of Table 4 below).

In the case of the greases which used other thickeners B to E together with thickener A (a lithium soap) in Examples 9 to 13 of Tables 2 and 3, the greases made as described below using thickeners B to E were specially prepared, and were mixed in a kettle specially used for preparation of greases at room temperature in the proportions shown for the thickeners in Tables 2 and 3. The various fatty acids and fatty acid metal salts were pasted in and, after a homogeniser treatment, the grease compositions for use in resin lubrication for each of Examples 9 to 13 were obtained.

In the case of the greases which used thickener B (a urea), the base oil, thickener B and the various fatty acids or fatty acid metal salts which are the additives were first weighed out in the proportions shown in Tables 2 and 3 so that the total amount of the grease composition would be 1000 g. Then part of the base oil and the MDI (4,4′-diphenylmethanediisocyanate) which was the raw material for thickener B were pasted into a kettle of capacity 3 kg specially used for preparation of greases. While heating and stirring, the temperature was raised to 60° C., and a reaction was effected by folding in octylamine already dissolved and mixed in the remaining part of the base oil. The temperature was further raised to 180° C., cooling was then effected at a fixed rate, and the aforementioned various fatty acids or fatty acid metal salts were pasted in and, after a homogeniser treatment, the grease compositions were obtained.

In the case of the greases which used thickener C (a bentonite), the base oil, thickener C and the various fatty acids or fatty acid metal salts which are the additives were first weighed out in the proportions shown in Table 3 so that the total amount of the grease composition would be 1000 g. Then the base oil, the bentonite of thickener C and an organic solvent to promote gelation were pasted into a kettle of capacity 3 kg specially used for preparation of greases. While heating and stirring, the temperature was gradually raised to 150° C. so that the organic solvent was made sufficiently volatile to effect uniform diffusion and swelling. Cooling was then effected at a fixed rate, and the aforementioned various fatty acids or fatty acid metal salts were pasted in and, after a homogeniser treatment, the grease compositions were obtained.

In the case of the greases which used thickener D (sodium terephthalamate), the base oil, thickener D and the various fatty acids or fatty acid metal salts which are the additives were first weighed out in the proportions shown in Table 3 so that the total amount of the grease composition would be 1000 g. Then the base oil and the methyl N-octadecylterephthalamate which was the raw material of thickener D were pasted into a kettle of capacity 3 kg specially used for preparation of greases. While heating and stirring, a suspension of sodium hydroxide already stirred and dispered in water was folded into the kettle at a temperature of 90° C., and a reaction was effected while gradually heating and stirring until the temperature reached 170° C. Cooling was then effected at a fixed rate, and the aforementioned fatty acid metal salts were pasted in and, after a homogeniser treatment, the grease compositions were obtained.

In the case of the greases which used thickener E (a tricalcium phosphate), the base oil, thickener E and the various fatty acids or fatty acid metal salts which are the additives were first weighed out in the proportions shown in Table 3 so that the total amount of the grease composition would be 1000 g. Then, the base oil, the tricalcium phosphate and an organic solvent to promote gelation were pasted into a kettle of capacity 3 kg specially used for preparation of greases. While heating and stirring, the temperature was gradually raised to 150° C. so that the organic solvent was made sufficiently volatile to effect uniform diffusion and swelling. Cooling was then effected at a fixed rate, and the aforementioned fatty acid metal salts were pasted in and, after a homogeniser treatment, the grease compositions were obtained.

For Comparative Examples 1 to 6, the various raw materials were weighed out in accordance with the proportions shown in Table 4, and grease compositions were prepared by following the method described for the aforementioned examples.

The following measurements and tests were carried out in order to compare the characteristics and performance of the examples and comparative examples.

1. Penetration: measured in accordance with JIS K2220-7, 2. Dropping point: measured in accordance with JIS K2220-8. 3. Kinematic viscosity of base oils: measured in accordance with JIS K2283. 4. Friction tests: Bowden type friction tests were carried out. In other words, the friction coefficient between a resin (test material 1b) and a paired material other than a resin (test material 1a) was measured under the following test conditions using a Bowden friction test rig.

-   -   (1) Test material 1a: Material—Copper alloy ALBC2 and steel         S45C.         -   Dimensions—pin shape of outside diameter 5.0 mm and length             24 mm, the pin tip being a semi-spheroid of r=2.5 mm, and             the contact surface was machined to a flat area of             approximately 1.0 mm diameter.     -   (2) Test material 1b: Material—polyacetal resin (Delrin 500P         made by Dupont Ltd.)         -   and polyamide resin (66 Nylon/Amilan made by Toray Ltd.)         -   Dimensions—plate of length 200 mm, width 52 mm.     -   (3) Temperature: 25° C.     -   (4) Sliding rate: 1.0 mm/s     -   (5) Load: 870 g     -   (6) Surface pressure of contact surfaces: 10 MPa

A Bowden friction test was carried out on all the examples and on all the comparative examples for a polyamide resin and steel pairing, and tests were carried out selectively for a polyacetal resin and copper alloy pairing.

Test Results

These are as shown in Tables 1 to 4.

Discussion

The grease compositions for resin lubrication of Examples 1 to 13 all displayed the grease characteristics of a semi-solid and the penetration displayed moderate hardness values in the range 267 to 290, while the dropping point was also of a satisfactory nature at not less than 177 to 210° C. Also, the friction coefficients between a polyamide resin and steel in the Bowden friction test were 0.060 to 0.069, and the friction coefficients between a polyacetal resin and copper alloy were uniformly low at 0.062 to 0.066, so that it was evident that a satisfactory lubrication performance was displayed between various resins and materials other than resins such as steel and alloys.

On the other hand, the grease compositions of Comparative Examples 1 to 6 all displayed the grease characteristics of a semi-solid, and the penetration displayed hardness values in the range 265 to 281, while the dropping point was also of a satisfactory nature at 180 to 207° C., but the friction coefficients between a polyamide resin and steel in the Bowden friction test were 0.082 to 0.099, and the friction coefficients between a polyacetal resin and copper alloy were all high at 0.099 or 0.101, so that it was evident that they were all inferior to the examples of the present invention as regards the lubrication state between various resins and materials other than resins such as alloys or steel, and that no effect in improving lubrication performance was obtained.

From these results it can be seen that the grease composition for resin lubrication of this invention exhibits satisfactory lubrication performance.

TABLE 1 Example 1 2 3 4 5 (1) Base oil (mass %) Lubricating oil A 89.5 89.5 89.5 89.5 89.5 Lubricating oil B — — — — Lubricating oil C — — — — — (2) Thickener (mass %) Thickener A 8.5 8.5 8.5 8.5 8.5 Thickener B — — — — — Thickener C — — — — — Thickener D — — — — — Thickener E — — — — — (3) Added amount of fatty acid 2.0 2.0 2.0 2.0 0.5 and/or fatty acid metal salt (mass %) Fatty acid (molar ratio) Caproic acid C6 — — — — — Caprylic acid C8 — — — — — Lauric acid C12 1 — — 2 — Myristic acid C14 — — 2 — — Palmitic acid C16 — — — — — Behenic acid C22 — — — — — Palmitoleic acid C16′ — 1 — — — Oleic acid C18′ — — — — 2 Erucic acid C22′ — — — — — Linolic aid C18″ — — — — — Metal (molar ratio) Magnesium — — 1 — — Calcium — — — — — Zinc — — — 1 — Aluminium — — — — 1 Total (1) + (2) + (3) 100.0 100.0 100.0 100.0 100.0 Penetration 278 275 278 277 272 Dropping point ° C. 181 177 179 180 179 Kinematic viscosity of base 101.1 101.1 101.1 101.1 101.1 oil (40° C., mm²/sec) Friction tests (friction coefficient) (1) Polyamide 0.061 0.062 0.068 0.069 0.066 resin-steel (2) Polyacetal 0.062 — — — 0.065 resin-copper alloy

TABLE 2 Example 6 7 8 9 10 (1) Base oil (mass %) Lubricating oil A 42.50 89.50 44.25 87.00 43.00 Lubricating oil B 42.50 — — — 21.50 Lubricating oil C — — 44.25 — 21.50 (2) Thickener (mass %) Thickener A 10.0 8.5 8.5 5.5 5.5 Thickener B — — — 5.5 5.5 Thickener C — — — — — Thickener D — — — — — Thickener E — — — — — (3) Added amount of 5.0 2.0 3.0 2.0 3.0 fatty acid and/or fatty acid metal salt (mass %) Fatty acid (molar ratio) Caproic acid C6 — — — — — Caprylic acid C8 — — 2 — — Lauric acid C12 — — — — — Myristic acid C14 — — — — — Palmitic acid C16 — — — — — Behenic acid C22 — — — — — Palmitoleic acid — — — — — C16′ Oleic acid C18′ — — — 1 — Erucic acid C22′ 2 — — — 2 Linolic aid C18″ — 1 — — — Metal (molar ratio) Magnesium — — — — — Calcium 1 — — — 1 Zinc — — — — — Aluminium — — 1 — — Total (1) + (2) + (3) 100.0 100.0 100.0 100.0 100.0 Penetration 281 276 280 289 290 Dropping point ° C. 178 178 178 210 207 Kinematic viscosity of 53.51 101.1 66.88 101.1 59.84 base oil (40° C., mm²/sec) Friction tests (friction coefficient) (1) Polyamide 0.067 0.060 0.068 0.064 0.067 resin-steel (2) Polyacetal — 0.062 0.065 0.064 — resin-copper alloy

TABLE 3 Example 11 12 13 (1) Base oil (mass %) Lubricating oil A 86.00 — 42.50 Lubricating oil B — 42.50 42.50 Lubricating oil C — 42.50 — (2) Thickener (mass %) Thickener A 6.0 6.5 6.5 Thickener B — — — Thickener C 6.0 — — Thickener D — 6.5 — Thickener E — — 6.5 (3) Added amount of fatty acid 2.0 2.0 2.0 and/or fatty acid metal salt (mass %) Fatty acid (molar ratio) Caproic acid C6 — — — Caprylic acid C8 — — — Lauric acid C12 — — — Myristic acid C14 — — — Palmitic acid C16 — — — Behenic acid C22 — — — Palmitoleic acid C16′ — — — Oleic acid C18′ 2 — 2 Erucic acid C22′ — — — Linolic aid C18″ — 2 — Metal (molar ratio) Magnesium 1 — — Calcium — — — Zinc — — — Aluminium — 1 1 Total (1) + (2) + (3) 100.0 100.0 100.0 Penetration 267 276 269 Dropping point ° C. 204 203 188 Kinematic viscosity of base oil 101.1 38.66 53.51 (40° C., mm²/sec) Friction tests (friction coefficient) (1) Polyamide resin-steel 0.064 0.066 0.063 (2) Polyacetal resin-copper 0.066 — 0.064 alloy

TABLE 4 Comparative Example 1 2 3 4 5 6 (1) Base oil (mass %) Lubricating 91.5 89.5 88.5 44.25 44.50 85.00 oil A Lubricating — — — 44.25 — — oil B Lubricating — — — — 44.50 — oil C (2) Thickener (mass %) Thickener A 8.5 8.5 8.5 8.5 5.5 6.0 Thickener B — — — — 5.5 — Thickener C — — — — — 6.0 Thickener D — — — — — — (3) Added — 2.0 3.0 3.0 — 3.0 amount of fatty acid and/or fatty acid metal salt (mass %) Fatty acid (molar ratio) Caproic acid — 1 — — — — C6 Caprylic — — — — — — acid C8 Lauric acid — — — — — — C12 Myristic — — — — — — acid C14 Palmitic — — 2 — — — acid C16 Behenic acid — — — 2 — 1 C22 Palmitoleic — — — — — — acid C16′ Oleic acid — — — — — — C18′ Erucic acid — — — — — — C22′ Linolic acid — — — — — — C18″ Metal (molar ratio) Magnesium — — — — — — Calcium — — — — — — Zinc — — — — — — Aluminium — — — — — — Total (1) + 100.0 100.0 100.0 100.0 100.0 100.0 (2) + (3) Penetration 273 275 281 278 265 267 Dropping 182 181 181 180 207 205 point ° C. Kinematic 101.1 101.1 101.1 53.51 66.8 101.1 viscosity of base oil (40° C., mm²/sec) Friction tests (friction coefficient) (1) Polyamide 0.082 0.099 0.093 0.088 0.096 0.096 resin-steel (2) Polyacetal 0.101 — 0.101 — 0.099 — resin-copper alloy 

1. A grease composition for use in resin lubrication, wherein the grease composition comprises: a grease base material, which includes a base oil and a fatty acid metal salt thickener, and an additive of at least one saturated or unsaturated fatty acid having from 8 to 22 carbon atoms and/or fatty acid metal salt, being a metal salt of a linear saturated fatty acid having from 8 to 14 carbon atoms or a metal salt of an unsaturated fatty acid having from 16 to 22 carbon atoms and from 1 to 4 unsaturated groups, the metal having a valence of from 1 to 4 excluding fatty acid metal salts used for the thickener.
 2. The grease composition for use in resin lubrication according to claim 1, wherein the metal of the aforementioned fatty acid metal salt is lithium, sodium, potassium, magnesium, calcium, zinc, aluminium or lead.
 3. The grease composition for use in resin lubrication according to claim 1, wherein the total content of the one or more saturated or unsaturated fatty acid and/or fatty acid metal salt is in the range of from 0.1 to 10% by mass relative to the total amount of the grease composition.
 4. The grease composition for use in resin lubrication according to claim 1, wherein at least one of a urea, bentonite, calcium phosphate or sodium terephthalamate is incorporated as a thickener together with the aforementioned fatty acid metal salt thickener. 